Laser perforating of carriers representing value is known per se in the art. For example, U.S. Pat. No. 5,975,583, the content of which is incorporated by reference in the present application, discloses a carrier representing value and comprising perforation patterns provided by a laser beam which are at least partially recognisable with the eye and which have such a structure that they cannot or only with the greatest difficulties be applied on the carrier by other processes. Such carriers representing value are generally known, for instance in the form of giro cheques, bank cheques, eurocheques, banknotes, credit cards, shares, bonds and other documents representing a value. This prior art also relates to other types of documents representing a value such as passports, driving licenses and the like. As indicated in this prior art patent publication, it is a known problem that carriers representing value can be forged and falsified. This is becoming an increasingly more significant problem. A constant attempt is made here to keep one step ahead of the forgers. In recent times the use of colour copiers has made it increasingly easier to forge documents which were otherwise difficult to counterfeit.
The laser device described in U.S. Pat. No. 5,975,583 comprises at least one laser source which is disposed such that the laser beam exits upwards through an exit aperture. The laser beam is then reflected by means of a mirror and deflected at an angle of 90°, passes through a shutter and is subsequently deflected downwards by another mirror. The laser beam then passes through a focussing device whereby focussing of the laser beam takes place. The laser beam then passes to another mirror whereby the beam is deflected and fed to a deflecting device. In the deflecting device the laser beam is carried to the relevant location on the paper where it performs the perforating operation according to the disclosed process. The device further comprises a detector which responds to reference marks arranged on the paper for generating a synchronization signal for the purpose of synchronizing the control of the laser beam with the movement of the paper. This is particularly important when the transport speed of the paper is not constant. More specifically, arranged in the focussing device is a lens which focuses the laser beam coming from the laser source on the position where the laser beam contacts the paper. Means are herein provided for moving the lens upward or downward to always keep constant the optical distance between the lens and the contact position, and thus keep the laser beam focussed on the contact position. The deflecting device is formed by a first galvanometer which is connected to a mirror with which the location of the contact position can be moved in the direction of movement of the paper, and a second galvanometer which is connected to a mirror with which the location of the contact position can be moved transversely to the direction of movement of the paper. With the described device any random perforation pattern can be applied to the paper.
Another prior art is known from US patent application No 2002/0027359 A1, the content of which is incorporated by reference in the present application, which relates to a security feature comprising a perforation pattern. In this publication, a document to be protected against forgery comprises a security feature in the form of a perforation pattern, wherein the perforation pattern extends over a surface of the document and represents an image comprising brightness tones. The perforation pattern is herein formed such that, for instance when the thus treated document is held up to the light or placed on a light box, an image becomes visible at the location of the perforation pattern. The arrangement of such an image representing brightness tones requires extremely advanced technologies. Such technologies are not easily accessible to potential forgers, so that documents thus provided with such a perforation pattern are very difficult to forge. In this publication, the perforation pattern is preferably applied by means of laser light.
Another prior art publication is the PCT application No WO 97/18092, the content of which is incorporated by reference in the present application. This publication relates to security documents with a security marking. More specifically, the disclosed security marking for security documents, in particular papers representing a value, consists of a plurality of circular or elongate holes, which are arranged in parallel rows in a printed area of the document. The diameter of the holes is chosen such that they are practically invisible with bare eyes in reflection, but become well visible when the document is held against a light and viewed in transmission. The holes are generated by laser pulses. The marking can be produced quickly and easily and it can be verified without technical aids.
A disadvantage of the known machines is that they are so-called stand-alone machines with their own independent sheet feeder, sheet transport system and delivery system.
Another disadvantage of the prior art machines is that the proposed perforation principle cannot directly be applied to high speed sheet-processing or printing presses such as those used in the production of securities, in particular of banknotes. With such high-speed presses, the sheets are transported at high speed (at a speed of about 10,000 sheets per hour), thereby inducing waves and deformations on the surface of the sheets that make it impossible to apply the perforation patterns onto the sheets with sufficient preciseness. This problem is moreover pronounced by the fact that, in such high-speed presses, the sheets are transported by means of a chain gripper system comprising a plurality of space-apart gripper bars each provided with a row of grippers to hold only the leading edge of a sheet. Accordingly, except for the leading edge of the sheet, the greatest part of the sheet is not as such supported or held in place, thereby making it impossible to apply the perforation patterns with sufficient preciseness.